Acid-hardening photoresist compositions that contain melamine resin as a crosslinker have been disclosed, for example in published European Patent Applications 85303807.3, 87300220.8, and 87300219.0. These patent applications disclose photoresists that contain acid-hardening resin that contain an aminoplast such as melamine resin as a crosslinker, along with resins such as novolac or polyvinylphenol, and a photoacid generator. The photoacid generator produces acid in response to exposure to the activating wavelength. The acid initiates the reaction between the components of the acid-hardening resin. The melamine resins that are disclosed to be useful in these applications include Cymel 303, a commercially available melamine resin that contains hexamethoxymethylmelamine resin monomer as well as dimers and trimers of the monomer.
Hexamethoxymethylmelamine, (HMMM), has been used in coating compositions and is commercially supplied, for example by American Cyanamid Company which markets products of various purity under the the tradename Cymel, such as Cymel 303, Cymel 300, etc.. The art contains disclosures of experiments that are said to be run using hexamethoxymethylmelamine when the reagent was a commercial resin such as Cymel. Commercially supplied melamine resin materials such as the Cymel resins contain significant quantities of impurities, such as dimers, trimers, tetramers, and contaminants. Gel Permeation Chromatography of samples of Cymel 300, a high purity version of hexamethoxy-methylmelamine, revealed: a monomer content of 74 to 79 percent by weight; a dimer content of from 16 to 19 percent; and a trimer content of from 4 to7 percent by weight.
Lithographic potential is a measure of the expected lithographic performance of a photoresist composition. Lithographic potential is expressed as a numeric value, equal to the Iog.sub.10 of the result of dividing the dissolution rate of the unexposed areas of the resist by the dissolution rate of the exposed areas of the resist. The lithographic potential is an expression of the observed degree of reaction in the exposed areas in response to the dose at a particular wavelength of the exposing radiation. A lowered dissolution rate indicates increased resistance of the resin to removal by the developer solution, and therefore serves as a measure of the degree of chemical reaction in response to the exposing radiation. The sensitivity of various resists can be compared by measuring their lithographic potentials. A resist that requires a lower dose at the exposing wavelength to produce a lithographic potential of a given value can be said to be more sensitive than a resist that requires a higher dose to produce a lithographic potential of the same value.